The present invention relates to vehicles having high pressure hydraulic circuits used for high pressure hydraulic devices such as hydraulic steering, brake valves, or hydraulic actuators, medium pressure hydraulic circuits used for a hydrostatic or geared transmission, and/or low pressure hydraulic circuits used for lubrication. More specifically, the present invention relates to work vehicles having high, medium, and low pressure hydraulic circuits.
Work vehicles are used in many different vocations, including agriculture, construction, industrial, and forestry, as non-limiting examples. These work vehicles can take many forms, such as backhoe loaders, wheel loaders, forestry fellers, skidders, harvesters, sprayers, application equipment, utility vehicles, agricultural tractors, and compact tractors. Often, these work vehicles have multiple hydraulic devices with differing requirements in terms of pressure and flow in order to function correctly. In order to provide for these requirements, a hydraulic circuit usually has multiple pumps of varying pressure and volume capacities, along with pressure relief or control valves, so that the demands of the multiple hydraulic devices are met at the appropriate pressures and flows.
More specifically, it is known to provide three levels of hydraulic pressure and flow in a hydraulic circuit of a work vehicle. A low pressure portion of the hydraulic circuit is used for lubrication. A medium pressure portion of the hydraulic circuit is used for a hydrostatic, continuously variable, gear, or other type of transmission, along with other miscellaneous medium pressure hydraulic needs of the work vehicle. A high pressure portion of the hydraulic circuit is used for hydraulic steering devices, brake valves, and for hydraulic actuators, including hydraulic actuators connected directly to the work vehicle, and including hydraulic actuators remotely connected to the work vehicle, such as those used on agricultural implements.
In order to accomplish this, known hydraulic systems used a non-adjustable positive displacement charge pump to supply hydraulic fluid at relatively low pressure and high volume. The low pressure high volume hydraulic fluid was then partially routed to a medium pressure pump that supplied the medium pressure needs of the work vehicle. The low pressure high volume hydraulic fluid was also partially routed to the lubrication circuits of the work vehicle by way of a hydraulic oil cooler and a pressure reducing valve. The low pressure high volume hydraulic fluid was also partially routed to a high pressure pump that supplied the high pressure needs of the work vehicle, also by way of the hydraulic oil cooler and pressure reducing valve.
While the non-adjustable positive displacement charge pump had a fixed displacement, the high pressure pump was routinely a closed center load sensing, in other words adjustable, pump that reacted to the demands of a priority valve supplying hydraulic pressure to the hydraulic steering devices, brake valve, and/or hydraulic actuators. The medium pressure pump may also have been variable displacement. As a result, much of the output of the non-adjustable positive displacement charge pump was often wasted, recirculating all flow not required at a particular moment by the lubrication circuits, the medium pressure pump, or the high pressure pump. Further, some of the output of the non-adjustable positive displacement charge pump that was in fact being used, required the use of inefficient pressure reducing valves due to the design of the known hydraulic systems. This wasted output and the unneeded pressurization, and possibly de-pressurization and/or re-pressurization, directly results in hydraulic noise, wasted engine power, fuel inefficiency, and unnecessary heating of the hydraulic fluid. Further, insofar as the lubrication circuits required hydraulic supply at a given moment, the non-adjustable positive displacement charge pump raised the hydraulic pressure unnecessarily high, requiring the aforementioned pressure reducing valve, and further contributing to power losses and heating.
Furthermore, the amount of hydraulic fluid passing through the hydraulic oil cooler was only passively controlled by the relative pressure drops across components of the hydraulic system, so that sometimes too little flow and often too much flow passed through the hydraulic oil cooler. This further resulted in hydraulic noise and inefficiency, along with insufficient or excessive cooling of the hydraulic fluid.
Presently, specific legislative requirements and general market demands require work vehicles to achieve reduced power consumption, increased fuel efficiency, reduced exhaust emissions, and quieter operation. Therefore, it has become increasingly important to minimize the aforementioned noise, wasted power, fuel inefficiency, and excessive heat generation.
What is needed in the art is a cost-effective and energy efficient hydraulic circuit for a work vehicle.